In medieval times, its color could symbolize ill omen or deceit.
The Loathly Damsel tells Arthur’s court that she cannot linger because by nightfall she must be far away at Castle Orgulous; this may suggest that her mount can travel faster than one might expect.
If the Loathly Damsel is indeed herself one of the marvelous ladies we find in the Arthurian landscape, who can wear either an ugly or a beautiful aspect, then her mule might be similarly marvelous.